This project investigates the functions of cytoplasmic motors in neurons in order to understand fast and slow axonal transport. Negatively charged macromolecular assemblies injected into the squid giant axon move in the anterograde direction at the slow rate. Of particular interest is that neurofilament proteins as well as actin and microtubule fragments move anterogradely along some type of intracellular tract. Recent results suggest that these tracts are microtubule bundles and that the motors come from the soluble pools of conventional kinesin. Indeed, depolymerizing the axonal microtubules blocks slow neurofilament transport. We were recently surprised to find that dimeric tubulin is also transported as well as diffusing. even under depolymerizing conditions. Thus, the slow motor applies to a wide variety of substrates ranging down to diffusible proteins which are given a directional bias. Myosin motors in squid axoplasm now appear to belong to several families of myosins and brain myosin II is likely to predominate on axoplasmic organelles. In the squid giant axon parallel actin filaments intertwine with the microtubule bundles. These have mixed polarities, suggesting that they might carry organelles to and from the microtubule bundles but they may also have a structural role. - axonal transport, organelle transport, kinesin, myosin actin, slow transport, flagellar motors